Like a Moth to a Flame

Thus hath the candle singed the moth.

–Shakespeare, The Merchant of Venice

Many of the oldest similes and metaphors feature an elemental timelessness. The notion of “a moth to a flame” connects us to our ancestors around fire rings before the advent of artificial light. Watching an insect inexorably drawn to a light source is an experience a human could have had in 4000 BCE and one you could have in 2024.

It’s easy to see why such a saying became anchored among human cliches. Watching a living being seemingly driven to its death against its biological imperative is certainly striking. The jump to human comparison seems natural. Any behavior a person undertakes compulsively, especially one detrimental to safety or happiness, and the person acts “like a moth to a flame.”

Since at least the time of Shakespeare – and likely much longer – we’ve utilized this phrase to great effect. The only problem with our employment over the centuries is we might have had the behavior of the moths all wrong.
Image generated by Kyle Stout

Whether fire or light bulb, insects seem unable to avoid artificial light. We’ve even developed bug zappers for the express purpose of luring them to their deaths. Many humans have experienced the rhythmic whomp of a moth or other large flier banging into an outdoor fixture during a summer night. The insects seem to be programmed to love these bright objects.

But why? Various theories have permeated entomology. Some scientists believed bright lights temporarily blinded insects. Others posited they are attracted to heat. Perhaps lights mimicked gaps in a dense forest, which the insects had evolved to find in case they needed to escape quickly. The most pervasive hypothesis centered around the moon. Maybe flying insects used our satellite to guide their movements and artificial light seemed to be a full moon. All these notions remained speculatory because testing flying insects remained nearly impossible.

A recent study, however, aimed to uncover the mechanism behind the destructive behavior thanks to some modern technology: motion capture cameras.

The trails of moths flying around a light bulb in super slow motion show the insects orienting their backs to the light.
The erratic flights of moths - photo by Fabian, Sondhi, Allen, and Lin

With the aid of some dyes and tiny cameras, scientists from the McGuire Center for Lepidoptera and Biodiversity at the Florida Museum of Natural History tracked the movement of moths flying around bright lights. The contraptions provided the best data ever on the behavior. When the researchers slowed the footage, they discovered something rather intriguing.

The moths weren’t so much divebombing the lights as they were circling them. When they did so, they tended to fly in odd paths. The video displayed the moths oriented their backs to the light source. For some reason, they automatically aligned their bodies to be “below” the source. When that source is a round bulb in the air, the result is a feedback loop for the moth.

This behavior seems to indicate the insects are not inherently drawn to the light itself as an object of desire or need. Instead, the lights interfere with a positioning mechanism or system.

The general behavior in moths is called “dorsal light response.” Over millions of years, the insects have developed the ability to remain aligned with the horizon so they can fly in straight, productive manners. For most of history, the natural starlight and moonlight in the sky alerted the moths about which way was up. Add artificial illumination and their natural systems can go haywire. If they spontaneously point their backs to the light source and that source is not starlight or moonlight, they might become “magnetically” attached to the source.

One scientist from the study, Sam Fabian, told Scientific American, “They are likely also using additional conflicting sources of information for which way is up; however, it looks like the direction of light overrules and dominates other cues. The visual system is saying, ‘No, you really need to keep that light over your back.’”

To further probe this behavior, the researchers decided to film flying insects with diffuse artificial light. Using a sheet, they spread out the source from a point to a larger area.

The insects did not become confused or erratic when they flew beneath the sheet.

Scientist Yash Sondhi quipped, “That very concretely showed that it’s not a light attraction issue but an orientation issue.”

A diffuse light set up in the woods to test moths flying without concentrated artificial sources.
Testing diffuse light - photo by Sam Fabian/NPR

This study points away from the old cliche’s implication that moths love something even though it will bring their doom. Nor do they think an artificial light is the moon or some other guiding lodestar. Instead, they are programmed to point their backs at a light.

Harvard entomologist Avalon Owens opined, “You get this sense that they don’t want to be doing it. Like they’re trying to…figure out how to navigate this weird situation, but they just don’t have the tools.”

As Fabian pointed out, “Insects have been flying around for 370 million years. It’s just in the last 150 years that it’s really gone wrong for them.” That’s a lot of evolution confronted with a new menace in the metaphorical flap of a wing. The more we learn about light pollution, the more widespread its issues become. We know it has a drastic effect on fireflies. Astronomers have urged humanity to keep nighttime lights to a minimum and, if necessary, to point them downward. This study indicates that aiming lights downward – keeping them from pointing up or sideways – can have a beneficial impact on moths and flying insects. Though they might become disoriented briefly thanks to an artificial source, if it points in a direction to which they are accustomed, they might be able to escape the death spiral.

As for the idiom, do we need to update our saying? The part of the phrase that relates to the inability to control one’s behavior remains intact, but the portion that likens one’s innate love of a harmful behavior might need an overhaul.

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